Communication systems, such as conventional telephone communication systems, provide audio communication between two or more users during a communication session. Each user may communicate with each other using a communication device having a speaker and a microphone. During a communication session, the communication device may experience echo (e.g., hybrid echo, acoustic echo, etc.).
The term “hybrid echo” (also referred to as “electric echo”) describes a phenomenon in which a fraction of the signal leaving the phone is reflected by a hybrid circuit and returns into the phone. This is particularly prevalent in voice-band communication circuits where there are impedance imbalances in local two-wire to four-wire hybrid circuits. The effect of hybrid echo is that the near-end user hears their own utterances repeated back to them. The echo delay of hybrid echo is often short enough that it is not perceived on analog phone handsets; however, hybrid echo can be more of a problem in speaker phone systems.
The term “acoustic echo” describes the phenomenon in which a local audio loop occurs when a microphone picks up audio signals from a speaker. Within a communication device, a speaker enables local reproduction of audio signals from the far end, and the microphone measures sounds produced at the near end. In this setting, acoustic echo occurs through mechanical and acoustic coupling (reverberation in the room or enclosure) between the speaker and microphone. The effect of acoustic echo is that the person at the far end hears a delayed version of their own utterances. Acoustic echo may be intensified by the speaker volume turned up to a high level and/or when the microphone and speaker are close together.
Beside the perceived unnaturalness of hybrid echo and acoustic echo, echo is actually a manifestation of a positive feedback in the communication system. Under suitable conditions, which are not under user control and which are random in nature, the communication system may become unstable. Echo cancellation systems may be employed within communication devices to cancel hybrid echo and/or acoustic echo.
A conventional echo canceler uses adaptive filtering algorithms to update its impulse response w(i) over time as new samples of the audio signals to and from the far-end become available. When the incoming far-end signal is inactive and the near end signal is active, then minimization of the variance of echo canceled signal e(i) leads to the matching condition of the impulse response w(i) and the echo path h(i). In other words, w(i)=h(i). Effectively, the adaptive filter has learned the impulse response of the physical echo path through the hybrid circuit. When both far-end and near-end signals are simultaneously active (i.e., a condition referred to as double talk) then the far-end signal acts as interference, the impulse response w(i) diverges from the echo path h(i), and residual echo becomes audible.
To prevent this divergence and maintain a lock on the matching condition, conventional echo cancelers employ double-talk detection strategies to control (start, stop and restart) adaptive updates. The main drawbacks of the conventional approach are twofold. First, double-talk detection may be an error prone process and inevitable missed detections of double talk lead to filter divergence and residual echo. Second, if the echo path h(i) changes while adaptation is halted, then the impulse response w(i) may not satisfy the matching condition and residual echo may occur.